The Throwback Galaxy is a galaxy from Super Mario Galaxy 2 which is a redesigned Whomp's Fortress
from Super Mario 64 and Super Mario 64 DS and showing that it is a throwback (as evidenced by the name). The music is a jazzy arrangement of the original Super Mario 64 main theme, as the original version was also played on Whomp's Fortress. Although the course is generally identical, a few minor changes have been made.

The clouds in the background are notably darker than the original Whomp's Fortress, and black holes were added below the course. New enemies have been added, such as Paragoombas, Skeeters and a Wiggler. Other enemies seen first in the original game also return, like Piranha Plants, Thwomps, Whomps, and Bomps. Some Bob-omb Buddies also appear in the galaxy, and even make reference to how Mario seems familiar, a reference to his earlier visit to Whomp's Fortress in Super Mario 64 and its remake. A Whomp is found at the top of the area, and defeating it will open an area where the Whomp King is found. A Comet Medal, Rainbow Notes (released from breaking a crate, playing the Super Mario Bros. underground theme) and a bonus planet featuring Piranha Plants can be found in the stage too. Several Cloud Flowers also appear in the galaxy's second mission.

A part of Whomp's Fortress. Notice that the walls are chipped away from the mission "Blast Away the Wall" from Super Mario 64.

The very start of the level.

A planet almost identical to Whomp's Fortress in Super Mario 64. However, the floating islands that contained Red Coins and the area where the Star Switch mission is done in Super Mario 64 DS are no longer found. The cannon used to destroy two walls in a mission in the previous games is replaced with a Sling Star, along with the points in the walls already being destroyed. There are a lot of enemies, such as Thwomps, and Piranha Plants in this planet, and at the top, Mario can defeat a Whomp to access a vertical tunnel which will take him to the underside of the planet where the Whomp King is fought.

There is a teleporter near the base of Whomp's Fortress that leads to the Bonus Planet, which is inhabited by multiple Piranha Plants. If Mario is able to defeat them all within the time limit, he will receive three 1-Up Mushrooms.

During the second mission, the player can use a Cloud Flower to get to the Bonus Planet, which is only a short distance away from the main planet. However, the player will have used all three of their clouds at this point, so they won't be able to get back again.

The next Green Star has the player redo Return of the Whomp King, like the first Green Star. It is located on top of the fortress, Mario/Luigi need to get on top of the Whomp, ride it to the Star, and then perform a Backwards Somersault to reach it.

When the Whomp King is defeated in Super Mario Galaxy 2, he will explode and Mario will be standing on the ground, while in Super Mario 64, Mario will still be in the air after the Whomp King is defeated.